Sugar may be produced from a variety of raw materials such as sugar-cane, sugar-beets and saccharine sorghum, for example, which all present common problems with color reduction.
In sugar production processes based on sugar-cane, the cane is crushed, with the raw juice resulting from the crushing stage being treated by the following stages: heating, sulfitation, liming, phosphatation and decantation, among others. The purpose of these stages is to reduce the color of the juice and also eliminate insoluble substances.
Next, the juice is concentrated in one or more evaporators. The resulting syrup is then boiled and allowed to crystallize in a crystallization pan and centrifuged, thus producing sugar.
As is well known by a person skilled in the art, the efficiency of the sugar production process and color removal depends greatly on the quality of the crushed cane. In turn, sugar cane quality varies according to climate conditions, soil types, sugar cane varieties, maturity levels, length of time between burning, harvesting and crushing, loading techniques, transportation and the operating conditions of the sugar mill.
A factor that directly affects sugar quality is color. Thus, during production, it is important to remove color, turbidity and solids in suspension in order to produce pale, high grade sugar. The quality of the sugar cane and its variations, depending on the characteristics mentioned above, may result in darker coloring that must be removed.
Under any conditions, sugar cane has a variety of natural pigments that gives color to the sugar produced, such as flavonoids, which are soluble and pass through the traditional process without being removed.
The amount of colored matter in sugar-cane juice is very low in terms of percentages, making only around 3% of non-sugar organic compounds in the juice. However, these compounds become more important due to their pronounced effect on the coloring of the juice and the end product: sugar.
As mentioned above, sugar production processes comprise several stages focused on color reduction, such as the heating stage, which helps with decolorization in three ways simultaneously: (a) when heating the juice resulting from the crushing process, undesirable micro-organisms are eliminated from the process and the juices consequently are sterilized; (b) during the juice color reduction stage (in the decanter), when heating the juice above its boiling temperature, gases and bubbles are eliminated from the sugar cane juice, making the decanter operation more efficient; and (c) higher temperatures are needed for chemical reactions to occur during processes, such as sulfitation, which is a stage also related to color reduction.
During the sulfitation stage, sulfur dioxide (SO2) acts on the juice by means of oxidation, reducing its color through a reaction that prevents darkening between the reduction sugars and amino acids.
The liming stage (pH correction) is also intended to reduce the color as it reduces saccharose losses through inversion, sterilizing the juice and potentiating the effects of the polyelectrolytes to be dosed. The product most widely used as a pH correction technique is lime.
Decantation is the stage in the juice treatment process where color reduction is more visible. At this stage, color reduction occurs through mechanical actions, with the juice becoming paler through removal of insoluble compounds in a decanter. In order for this concentration of insolubles to occur, in addition to the liming stage, it is necessary to introduce polyelectrolytes, generally at the input point of the decanter, which neutralize the electrostatic repulsion forces among the particles, thus ensuring coagulation and flocculation.
It is common to find sugar cane juices that are deficient in phosphate, reducing their decanting capacity. Thus, it is a common practice to introduce phosphoric acid, for example, as phosphate supplement. The calcium phosphate crystals are precipitated out along with colored bodies. This stage is known as phosphatation.
In addition to the stages mentioned above, depending on the quality of the sugar to be produced, other stages may also be added to the process in order to reduce the color, such as syrup flotation, magma remixing and refining.
Syrup flotation reduces the color and turbidity, by removing insoluble solids from the syrup that are suspended on the surface of the mixture, which are removed through mechanical means.
At the magma remixing stage, the magma is recycled back through continuous centrifuges, removing more mother liquors. It must be noted that the mother liquor is the darkest compound with the lowest purity in the process.
It is worthwhile stressing that syrup flotation and magma remixing are processes that, in addition to requiring the installation of new equipment, outlays on input materials, heat energy and electricity, also reduce the recovery of sugar in the mill.
The refining stage is used to obtain refined and/or paler sugar, basically encompassing three operations: (a) dissolving the sugar to obtain liquor, and heating, (b) flotation of the liquor in a clarifier, with removal of undesirable materials from the surface, and (c) recrystallization. Color reduction occurs during the liquor flotation stage, which works on the same concept as syrup flotation as described above.
The high chemical, energy and equipment cost of the refining stage substantially affects the sugar processing efficiency and profitability.
As described above, all traditional processes and known techniques for removing colored impurities during sugar production use physical means (such as decantation, heating and centrifuging) to reduce color of the sugar produced. The few chemical processes used, such as sulfitation and phosphatation, for example, may result in mass losses during the sugar production process.
Furthermore, the chemical and physical processes which are part of the state of the art are not efficient. Traditional sugar mills are unable to easily produce crystal sugar with 150 UI (ICUMSA units) of color as established by the International Commission for Uniform Methods of Sugar Analysis through the processes for reducing color described above. The sugar produced by these mills, known as Very High Polarization (VHP) sugar has up to 1000 UI. It must be stressed that even if low color sugar is achieved during production, the color of the sugar crystals increases over time.
It must also be considered that all the difficulties mentioned above may become even more significant, depending on the sugarcane variety, which is one of the factors with the strongest influence in the color reduction process. There are some varieties of cane, such as RB92579, that are highly productive but result in highly colored syrup. This color cannot be efficiently reduced through the existing processes described above.
Consequently, there is a need for sugar color reduction methods in sugar production processes whose output is stable, which might even shorten some stages of the process, and which are economically feasible.